Field of the Invention
The present invention relates to an insert nut, and more specifically, to a metal insert nut having an internal thread, which is to be inserted into and integrated with a hole portion of a mating plastic member, to thereby enable a screw clamp. In addition, the present invention relates to a fastening unit including the insert nut, and a process cartridge using the fastening unit.
Description of the Related Art
Hitherto, usage of a screw for resin has been known for carrying out screw fastening to a resin. However, when the screw for resin is repeatedly installed and removed, the resin is known to be plastically deformed, which may cause damage on an internal thread. In order to prevent this problem, there is known a method of mounting a metal insert nut into a resin component in advance, to thereby secure the thread strength.
As a configuration of such an insert nut, for example, a configuration as disclosed in Japanese Patent Application Laid-Open No. 2010-48308 is known. As in Japanese Patent Application Laid-Open No. 2010-48308, there is known an insert nut having a flange at the top surface. Hitherto, the insert nut has been provided with a flange so that, when a metal terminal is screw-fastened for earthing or the like, the metal terminal is securely sandwiched between the screw and the insert nut.
Further, in order to suppress loosening of the screw after screw fastening due to vibrations of the apparatus, as in Japanese Patent Application Laid-Open No. H11-247817, there is known a screw whose head has a flange formed into an umbrella shape (dish facing-down shape), which is widely used.
However, it has been found that the following problem occurs when the related-art insert nut is used. When the above-mentioned flanged screw is used for the flanged insert nut as disclosed in Japanese Patent Application Laid-Open No. 2010-48308 so as to enhance the fastening force with respect to a member to be fastened, in some cases, the insert nut is jacked up from a substrate during fastening, or the screw is loosened along with use. This is caused due to a difference between the flange diameter of the insert nut and the head diameter (flange diameter) of the screw, which is now described with reference to FIG. 6. FIG. 6 is a view illustrating a state of screw-fastening a member 68 to be fastened with use of an insert nut according to a comparative example (in FIG. 6, for easy understanding, the flange of the insert nut is illustrated large in a radial direction in an exaggerated manner). In general, the insert nut to be used in an image forming apparatus has a size of M3 (screw outer diameter of 3 mm) or more in order to obtain a predetermined breaking strength. In this case, the flange diameter of the flanged screw is substantially about 8 mm. Hitherto, the flange diameter of the insert nut has been about 5.5 mm. Therefore, as illustrated in FIG. 6, such a relationship that an outer diameter of a flange 64 of a screw 57 is larger than an outer diameter of a flange 3 of an insert nut 1 has been obtained. Due to this difference between the flange diameters, when the screw is fastened, a force (extraction force) of pulling out the insert nut 1 from the substrate strongly acts. When the extraction force exceeds the fixing force generated between the insert nut and the substrate, problems such as extraction of the insert nut from the resin and loosening of the screw may occur. Such problems may occur not only in the case of using the flanged screw but also in the case of using such a general-purpose screw that there is a large difference between the flange diameter of the insert nut and the outer diameter of the screw in a region where a seating face is brought into contact with a member to be fastened.